The invention relates to novel photochromic transparent organic materials with high index of refraction presenting no optical distortion in solid form, a method for their preparation, and the articles made from these materials.
It is difficult to prepare such a photochromic ophthalmic lens made of plastic material. Ideally, the polymer matrix of such a lens should be crosslinked thermally, free of optical distortion in solid form and able to receive appropriate mixtures of photochromic dyes and to be crosslinked in the presence of such appropriate mixtures of photochromic dyes, for example spiroxazines and chromenes, to obtain a material which presents a high initial transmission before exposure and a high capacity for darkening after exposure, all with rapid darkening and lightening kinetics. In addition, the material should have a low thermal dependence, a high resistance to fatigue, and as high an index of refraction as possible.
Obviously, no such ideal material exists to date.
WO-A-92/05209 describes a copolymer that is free of optical distortions and is suitable for the manufacture of eyeglasses, and is obtained by a radical polymerization of a first monomer with formula I ##STR3## where a and b are whole numbers from 0 to 4, R.sub.1 and R.sub.2 =H or CH.sub.3, and R.sub.3 =--O--, --S--, --CO--, --SO.sub.2 --, --CH.sub.2 --, --CH.dbd.CH-- or ##STR4## with a second monomer, which can be styrene or a styrene derivative an, optionally, a third monomer, which can be a vinyl aromatic compound or an aromatic methacrylate, in the presence of an initiator of the peroxide type and of a chain transfer agent selected from various brominated compounds.
This copolymer could be a good polymer matrix for an ophthalmic article, such as a lens.
However, when one tries to incorporate ophthalmic dyes in the copolymerizable composition to impart ophthalmic properties to the matrix obtained after copolymerization, it can be observed that said dyes are destroyed by the peroxide initiator.
Even if the peroxide initiator is replaced by a milder radical polymerization initiator, such as a diazo compound, for example, azobisisobutyronitrile, the material obtained is still unsatisfactory, because the photochromic dyes are inhibited by the bronninated chain transfer agent used, as has been observed by the present inventors. It is also necessary to later incorporate the dyes into the matrix notably by a specific thermal diffusion method, which increases the cost price of the material and complicates its manufacturing process.
Thus, there is a need for high-performance ophthalmic materials that are easy and inexpensive to manufacture.
The present invention is intended to meet this need.